Emulsions have been widely used in beverage technology, cosmetics, or pharmaceutical formulations for many years. Nevertheless, their limited thermodynamic stability, which means that they separate into their two original liquid phases on standing, represents the biggest drawback in application. Due to their time limited thermodynamic stability, all the emulsion based products will undergo oil-ringing formation, and finally phase separation.
Unlike emulsions, the microemulsions, including the micellar solutions, are usually transparent dispersions that form spontaneously without the need of energy input, when the compounds thereof are properly mixed with each other. Due to the very small size of dispersed oil-droplets, in fact in most cases the dispersed oil-droplets are less than 140 nm in diameter, the visible light cannot be scattered and therefore microemulsions appear as clear or translucent isopropic solutions. A classical oil-in-water microemulsion consists of water, a co-solvent such as alcohol, oil and one or more surfactants and co-surfactants. Although microemulsions form spontaneously, when they form, the selection of the components thereof and their relative amounts are very critical for their formation, their final characteristics such as optical appearance, and their organoleptic and thermodynamic time-stability. In addition, when such microemulsions are used as flavor delivery systems in food products, for example in beverages, they must fulfill all the requirements of these products, namely an excellent shelf-life stability over a large temperature range, at least a few months, and no formation of off-notes.
In the field of beverage flavoring, the use of microemulsions has been described in particular by Wolf et al. in U.S. Pat. No. 4,835,002. The latter document discloses microemulsions of edible oils in a matrix of water and certain alcohols, together with food-grade surfactants. More particularly, this document describes compositions comprising from 0.01 to 45% by weight, and preferably from 1 to 25% by weight, of oil, from about 0.1 to 60%, and preferably from 1 to 30% by weight, of surfactant and from 20 to 95%, and preferably 25 to 80% by weight, of a polyol including ethanol, propylene glycol, sugars such as dextrose, sucrose, fructose and other. The proportion of alcohol needed in said microemulsion compositions is of at least 20% by weight in order to obtain a clear system. Propylene glycol is selected as preferred alcohol.
Now, one objective of the present invention is to prepare a high oil loading, transparent, microemulsion with an optimized ratio between the surfactant system and the alcohol.
U.S. Pat. No. 5,283,056 describes compositions of transparent oil-in-water microemulsion concentrates, consisting essentially of water, one or more hydrophobic flavor or fragrance oils and one or more surfactants. Said microemulsions are especially intended for the preparation of mouthwash solutions and are essentially free of lower alkanols. Nevertheless, these compositions comprise large amounts of surfactant, particularly when high oil content is required. Moreover, the claimed microemulsions show poor shelf-life stability even at room temperature, that would not be convenient for beverage applications.
We have now been able to establish a novel and advantageous formulation of edible, perfectly clear, high oil loaded microemulsions that are entirely composed of food grade quality components, which comprise a very low level of surfactant/co-surfactant, and which prove excellent organoleptically and from the point of view of the thermodynamic stability, both as emulsions end and in end-products where they may be employed as flavor carriers.